Visual excitation is initiated by the absorption of a photon by the ll-cis retinal chronophoric group in the visual pigment, rhodopsin. This event is followed by a sequence of conformational changes in the visual protein and by the cis yields trans isomerization of retinal. The objective of this proposal is to determine in detail the molecular mechanism for these events through the use of resonance Raman spectroscopy. This objective will be approached in the following ways: (1) We will construct a time-resolved resonance Raman spectrometer that is capable of observing transient species with lifetime as short as 30 nsec. This will enable us to obtain the room temperature Raman spectra of the two earliest photolytic intermediates following the excitation of rhodopsin. The comparison of these spectra with Raman spectra of model compounds should reveal when the complete cis yields trans isomerization has occurred and whether any intermediate conformations or interactions with the surrounding protein are important in the isomerization of retinal. (2) This comparison will be facilitated by vibrational normal mode calculations which will be developed through Raman studies of chemical and isotopic derivatives of retinals. (3) The conformational predictions of these calculations for the visual pigments will then be tested through Raman studies of visual pigments which have had their retinal chromophore or protein structure slightly altered. The chromophore analogs will be obtained by complexing visual pigments with isotopically or chemically altered retinals. The protein analogs will be obtained by studying visual pigments with different functional properties from other animate sources.